Welding Prep in Pressure Vessel Fabrication

Tony Hufford - Category Manager, Welding & Fabrication, Weiler

This article first appeared in the November 2015 issue of The Fabricator

The welds in pressure vessel fabrication have to be of the highest quality. The proper surface preparation tools and techniques can help to ensure those critical welds pass the toughest inspections.

Pressure vessels are used in many industries to store and transfer gas or liquid under high pressure. Because of the critical nature of pressure vessel applications, their welds undergo rigorous quality checks, which can include X-ray inspection and certification.

If a weld fails to meet the requirements of a nondestructive test, costs escalate. The weld must be removed, and the area must be appropriately patched. This is an expensive process that can cause a project to miss its expected outcomes for cost and deadline. As a result, proper welding techniques, including surface preparation, are critical to achieving profitable projects and repeat customers.

PRESSURE WELD INSPECTION PROCESSES

ASME (American Society of Mechanical Engineers), ANSI (American National Standards Institute), and ASTM (American Society for Testing and Materials) standards govern pressure vessel welds. Two common types of inspection for pressure welds are X-ray inspection and ultrasonic inspection.

With X-ray inspection, subsurface cracks and inclusions can be detected. This is an expensive process, but to ensure safety in critical weld joints (such as those found in submarines and nuclear power plants), a 100 percent X-ray examination is typically conducted.

The ultrasonic inspection also can be used to detect surface and subsurface defects. This process involves directing a high-frequency sound beam through the base metal and the weld on a predictable path. When the beam strikes a discontinuity, some of the sound beams are reflected. This reflected beam is received, amplified, and processed. When the time delay is used, the location of the flaw is estimated.

Each of these nondestructive testing methods has advantages and disadvantages that need to be weighed based on the type of job and the applicable standard. Regardless of which testing method is used, certain types of weld defects are serious problems and must be assiduously avoided.

COMMON CONCERNS

When an inspection uncovers a weld defect, it is usually one of several varieties. Here is a look at some common defects that can cause welds in pressure vessels to fail the inspection and some best practices for preventing them.

Porosity is caused when gas is trapped in the molten weld pool. As the weld cools and solidifies, the gas forms bubbles that appear as voids upon inspection. Numerous issues can cause porosity in a weld. It’s important to check that proper welding techniques are followed and appropriate consumables are being used.

Proper preparation and cleaning of the weld surface before welding and between welding passes are also important to help prevent porosity. This is particularly true in applications involving aluminum. During aluminum welding, any liquid hydrocarbons in the weld area are a potential source of hydrogen, which will readily dissolve into an aluminum weld pool. When the weld solidifies, the hydrogen atoms will collect and form small pockets of trapped gas, or porosity. Ensuring that the weld area is clean and dry is the best way to prevent this problem.

After-rust is light rust-colored surface contamination that appears in a stainless steel weld zone several hours or days after welding. It is the nemesis of shops and contractors that work with stainless steel.

The most common source of after-rust is cross-contamination. This typically occurs when a grinder with a steel brush is used to clean a stainless steel weld.

However, it can also be caused by cleaning the surface with a stainless steel brush previously used to clean steel. The magnetic characteristic of a stainless steel brush will cause it to collect steel particles, which will be deposited on a stainless steel surface and can result in after-rust. The best way to address this issue is to keep stainless steel brushes in tightly closed containers and to never use them for steel applications.

The third source of after-rust is a metallurgical phenomenon that results from severe cold-working of a surface during cleaning with a brush. If after-rust is encountered and cross-contamination can categorically be eliminated as the source of the problem, a potential solution is to switch to a brush with finer wire and use it with light pressure. This will eliminate the deleterious effects of cold-working that can be caused by very aggressive brushing.

Nitrides are a highly adherent contaminant created when plasma cutting with compressed air or nitrogen. They make the edges brittle and create porosity in some welding processes, especially gas metal arc welding. Because nitrides can exist from 0.005 to 0.010 inches below the surface of the material, you cannot remove them with brushes. Also, nitrides often cause bonded abrasive grinding wheels to load and subsequently smear. Flap discs or flap wheels work well for this issue because they are aggressive enough to remove a small amount of base material, but the grain and cloth of the accessories wear down at a rate that resists loading.

Inclusions often result from surface contaminants that become mixed into the weld pool and are trapped during solidification. In multipass welding applications, slag that is not completely removed can be a source of inclusions. Thorough cleaning with a suitable wire brush before welding and between passes is a very effective means of eliminating this type of defect.

This list is only a subset of the universe of weld defects. These are the issues that most frequently arise from inadequate or inappropriate surface cleaning. Other common defects—such as stress cracking and poor penetration—must also be controlled, but they have limited, if any, connection to surface condition. Proper welding parameters and procedures are critical to controlling these problems.

SELECTION AND OPERATING TIPS

When it comes to selecting and using the right products for an application, a few general tips should be kept in mind:

Industrial-grade, heavy-duty wire brushes quickly and efficiently remove surface contamination that causes defects and is ideal for many pressure vessel applications. Be sure to match the type of brush to the type of material being welded. A stainless steel brush should be used to prepare stainless steel materials, for example.

Use the largest brush that space will allow. A larger brush reduces the cost of a cleaning operation by reducing cleaning time and increasing consumable life. Choosing a knotted- or crimped-style brush depends on the adherence of the surface contaminant. Knotted brushes are best for applications requiring maximum aggression. Crimped brushes provide superior conformability to irregular surfaces.

Applying the right amount of pressure when using the wire brush is also important to maximize cleaning action. With a wire brush, the tips of the wires are designed to do the work, and the appropriate amount of pressure is just the weight of the tool itself. You should not need to push down hard to get a wire brush to work efficiently. If you must apply significant pressure to accomplish the job, you probably need a different brush.

The Big Payoff

The old saying “If it’s worth doing, it’s worth doing right? is widely applicable, and it’s especially suitable in the context of surface preparation for highly critical welds. Rework costs can be very high, so good cleaning procedures should be considered an investment that will consistently pay the dividend of weld reliability. For shops and contractors who survive on the quality of their welds, this is an investment that pays off.